Modeling Seismic Velocities for Exhumed Rocks

These tutorials were developed as part of an NSF-funded workshop (EAR-2419991) that taught a range of attendees (from undergraduate students to faculty) how to calculate the seismic velocities from exhumed rocks. We envision that these tutorials could be used in a range of courses (e.g., as a case-study project in an upper-level undergraduate geophysics course, a Matlab-focused course, or a graduate-level senior seminar on topics such as microstructures, geodynamics, and/or subduction processes) or as stand-alone exercises that could be incorporated into onboarding for students conducting research in these areas.

Students should have some basic prior knowledge about mineralogy, ductile structures (including lineation and foliation) and basic ductile deformation mechanisms. We also assume some basic Matlab skills, including setting directories, editing scripts, calling functions, and plotting, but theOverview file below has resource suggestions for learning those skills, as needed.

This could be used as a stand-alone exercise or as a starting point for a culminating project where students learn the basics through the tutorials and then choose a set of exhumed rocks to model. The tutorials provide suggestions for future applications and workflows that could form the basis for a project.

After completing these tutorials, students will be able to:

• Explain how lithology, P-T conditions, and mineral anisotropy affect seismic velocities.
• Calculate seismic velocities for isotropic and anisotropic media by estimating mineral and lithologic modal abundances, and mineral orientations for a given area of interest.

These tutorials support higher order thinking as students apply the methods to their own case study, including:

• Selecting appropriate mineralogical and lithological characteristics based on field and/or lab work.
• Constructing a mathematical model of complex deformed rocks.

Through the exercises, students are challenged to think spatially and to learn and/or improve their computer skills, including:

• Reading a seismic velocity pole figure in the context of the geologic setting.
• Determining 3D spatial orientations of minerals based on seismic velocity patterns.
• Learning and applying a programming language (Matlab).

Geophysical data provide insights into processes happening below the surface of the Earth. Exhumed rocks also provide a window into processes, but research on exhumed rocks is usually at a smaller scale (thin section to outcrop) than what geophysical data capture (commonly km-scale). One way to bridge these data is to model the seismic velocities of the exhumed rocks and compare the results to geophysical data from comparable settings.

Researchers have developed multiple toolboxes (Abers & Hacker, 2016; Walker & Wookey, 2012) to facilitate calculation of seismic velocities in complex rocks. These include adjustments for pressure-temperature (P-T) conditions and accounting for mineral anisotropy, where the seismic wave velocity depends on how the wave passes through a mineral. This is especially useful for ductilely deformed rocks, where mineral crystal lattices may be aligned during deformation.

These tutorials walk students through the basics of seismic velocities and how to use these two Matlab toolboxes to model seismic velocities for exhumed rocks. The students complete an overview and three parts at their own pace:

• Overview
  • SeismicVelocityModellingOverview.pdf (Acrobat (PDF) 83kB Sep30 26)
  • SeismicVelocityModellingOverview.docx (Microsoft Word 2007 (.docx) 25kB Sep30 26))
• Part 1: Basics of seismic velocities
  • Part1_SeismicVelocityBasics.pdf (Acrobat (PDF) 94kB Sep30 26)
  • Part1_SeismicVelocityBasics.docx (Microsoft Word 2007 (.docx) 25kB Sep30 26)
• Part 2: Assuming isotropic rocks
  • Part2_IsotropicRocks.docx (Microsoft Word 2007 (.docx) 450kB Sep30 26)
  • Part2_IsotropicRocks.pdf (Acrobat (PDF) 3.2MB Sep30 26)
  • Matlab script for this section: SeismicVelocityEstimates.m (Matlab File 21kB Sep30 26)
• Part 3: Accounting for mineral anisotropy
  • Part3_MineralAnisotropy.pdf (Acrobat (PDF) 5.6MB Sep30 26)
  • Part3_MineralAnisotropy.docx (Microsoft Word 2007 (.docx) 2.3MB Sep30 26)

The Overview and Part 1 include some background information on why these calculations might be useful in research and some basics on seismic velocities. If you are teaching this within a course and students have not learned about different types of seismic waves, the human seismic wave demonstration is a great option. Depending on the focus of your course, you might want to spend more time previewing geophysical data and conclusions prior to modeling exhumed rocks. This could include, for example, details on receiver functions and low velocity zones in modern subduction zones.

Students should be familiar with the basic functions of Matlab.The Overview describes what they should be able to do and provides suggestions for free resources that could be assigned as pre-work to teach the needed basic understanding.

These tutorials are designed to be used asynchronously, with students working at their own pace, so they include answers that students can use to check their work as they go. There are also notes on common errors and suggestions on what to check as the students are trying the exercises.

The tutorials provide multiple exercises throughout that could be submitted for assessment. We also recommend that faculty use this as a starting point for a summative project where students then model a region of their choice using the skills they learned from this tutorial.

These tutorials use two published Matlab Toolboxes:

• Abers & Hacker (2016) - available in the supporting information
• Walker & Wookey (2012) and MSAT toolbox